1. Field of the Invention
The present invention relates to an organometallic complex. In particular, the present invention relates to an organometallic complex that is capable of converting a triplet excited state into luminescence. In addition, the present invention relates to a light-emitting element, a light-emitting device, an electronic device, and a lighting device each using the organometallic complex.
2. Description of the Related Art
In recent years, there has been an active development of light-emitting elements in each of which an organic or inorganic compound having a light-emitting property is used as a light-emitting material. In particular, a light-emitting element called an EL (electroluminescence) element has attracted attention as a next-generation flat panel display element because it has a simple structure in which a light-emitting layer containing a light-emitting substance is provided between electrodes, and characteristics such as feasibility of being thinner and more lightweight and responsive to input signals and capability of driving with direct current at low voltage. Moreover, a display using such a light-emitting element has high contrast, excellent image qualities, and a wide viewing angle. Furthermore, such a light-emitting element is a planar light source, and accordingly its applications to light sources, such as backlights of liquid crystal displays and lighting, have been under contemplation.
In the case where the light-emitting substance is an organic compound having a light-emitting property, the emission mechanism of the light-emitting element is a carrier-injection type. Specifically, by application of a voltage to electrodes between which the light-emitting layer is interposed, electrons and holes injected from the electrodes recombine to raise the light-emitting substance to an excited state, and light is emitted when the substance in the excited state returns to the ground state. Possible excited states are a singlet excited state (S*) and a triplet excited state (T*). In addition, the ratio of S* to T* fainted in the light-emitting element is statistically considered to be 1:3.
In general, the ground state of an organic compound having a light-emitting property is a singlet state. Luminescence from a singlet excited state (S*), which is electron transition between the same multiplicities, is called fluorescence, and luminescence from a triplet excited state (T*), which is electron transition between different multiplicities, is called phosphorescence. At room temperature, observations of a compound which emits fluorescence (hereinafter referred to as a fluorescent compound) usually show only fluorescence without phosphorescence. Thus, the internal quantum efficiency (the ratio of generated photons to injected carriers) of a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% based on a S*-to-T* ratio of 1:3.
On the other hand, by use of a phosphorescent compound, the internal quantum efficiency can be increased to 75% to 100% in theory. In other words, an element using a phosphorescent compound can have three to four times as high emission efficiency as that of an element using a fluorescent compound. For these reasons, a light-emitting element using a phosphorescent compound has been actively developed in recent years in order to achieve a highly-efficient light-emitting element. As the phosphorescent compound, organometallic complexes that have iridium or the like as a central metal have particularly attracted attention because of their high phosphorescence quantum yield: for example, an organometallic complex that emits light in the wavelength range of green to blue is disclosed as a phosphorescent material in Patent Document 1.